1. Field of the Invention
The present invention relates to current excitation type light-emitting elements. In addition, the present invention relates to light-emitting devices and electronic devices which have the light-emitting elements.
2. Description of the Related Art
In recent years, research and development have been extensively conducted on light-emitting elements utilizing electroluminescence. In a basic structure of such a light-emitting element, a substance having a light-emitting property is interposed between a pair of electrodes. By applying voltage to this element, light can be emitted from the substance having a light-emitting property.
Since such a light-emitting element is a self-light-emitting type, it has advantages over a liquid crystal display such as higher visibility of pixels and unnecessity of a backlight. Accordingly, such a light-emitting element is considered suitable as a flat panel display element. In addition, other advantages of such a light-emitting element are that it can be manufactured to be thin and lightweight and the response speed is very high.
Furthermore, since such a light-emitting element can be formed into a film shape, surface light emission can be easily obtained by forming a large-area element. This is a feature that is difficult to be obtained from a point light source typified by a filament lamp and an LED or a linear light source typified by a fluorescent light. Therefore, the light-emitting element has a high utility value as a plane light source that can be applied to lighting or the like.
Light-emitting elements utilizing electroluminescence are classified broadly according to whether they use an organic compound or an inorganic compound as a substance having a light-emitting property.
When an organic compound is used as a substance having a light-emitting property, electrons and holes are injected into a layer containing the organic compound having a light-emitting property from a pair of electrodes by voltage application to a light-emitting element, so that current flows therethrough. Then, these carriers (electrons and holes) are recombined; thus, the organic compound having a light-emitting property is brought into an excited state. When the light-emitting organic compound returns to a ground state from the excited state, it emits light. Based on this mechanism, such a light-emitting element is referred to as a current-excitation light-emitting element.
Note that the excited state of an organic compound can be either a singlet excited state or a triplet excited state, and light emission from the singlet excited state is referred to as fluorescence and light emission from the triplet excited state is referred to as phosphorescence.
As for such a light-emitting element, there are many problems depending on materials in improving element characteristics, and improvement in element structure, development of materials, and the like have been conducted to overcome the problems.
For example, in Non-Patent Document 1 (Non-Patent Document 1: Tetsuo TSUTSUI and eight others, Japanese Journal of Applied Physics vol. 38, L1502 to L1504, (1999)), a hole-blocking layer is provided so that a light-emitting element using a phosphorescent material efficiently emits light. However, as described in Non-Patent Document 1, a hole-blocking layer has poor durability, and the light-emitting element has a very short lifetime. Thus, development of a light-emitting element with a long lifetime has been desired.